This application is based on and claims priority under 35 U.S.C. sctn. 119 with respect to Japanese Application No. 2002-217521 filed on Jul. 26, 2002, the entire content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a steering control system for a motor vehicle such as, for example, a motorcar.
2. Discussion of the Related Art
In the field of steering devices for vehicles and especially, in the field of steering devices for motorcars, for higher performance, there has recently been developed a system incorporating a so-called xe2x80x9cvariable steering angle ratio converting mechanismxe2x80x9d which is capable of varying the conversion ratio of a handle steering angle to a wheel steering angle (steering conversion ratio) in dependence on the traveling state of the vehicle without keeping the manipulation angle of a steering handle (handle manipulation angle) and a wheel steering angle fixed at a one-to-one ratio. For example, the vehicle speed can be exemplified as representing the vehicle traveling state, and in the system, the stability in a high speed traveling can be ensured by making the steering conversion ratio small so that the steering angle does not increase sharply with increase in the handle manipulation angle during the high speed traveling. During a slow speed traveling, on the contrary, the steering conversion ratio is made larger so that the number of rotations needed to make a full turn of the handle is decreased. This advantageously makes it possible that the manipulation with a large steering angle as required for, e.g., garaging, in-line parking, pulling-over to the kerb or the like can be performed very easily.
For example, as disclosed in Japanese unexamined, published patent application No. 11-334604(1999-334604), a mechanism for changing the steering conversion ratio is constituted by directly connecting a handle shaft and a wheel steering shaft with a gear type transmission unit whose gear ratio is variable. However, this type of the mechanism involves a drawback that the gear ratio changing mechanism of the gear type transmission unit is complicated in construction. Another type of the mechanism has been proposed in, for example, Japanese unexamined, published patent application No. 11-334628 (1999-334628), wherein a wheel steering shaft is rotationally driven by a motor. More specifically, a wheel steering angle finally needed is calculated through a computer processing based on a steering conversion ratio which is determined in dependence on a handle manipulation angle detected by an angle detection section and the traveling state of the vehicle, and the wheel steering shaft is rotationally driven by the motor with the handle shaft being disconnected mechanically therefrom thereby to bring the wheel steering angle into coincidence with that calculated.
In the foregoing steering control method, the frequency in operation of the motor for rotationally driving the wheel steering shaft increases very high due to the frequent handle manipulations performed during the driving of the vehicle. Accordingly, in improving the maintenance capability of the motorcar practicing the control method of this kind, it is important to ensure that the motor can operate stably over a long period of time. For example, motors tend to deteriorate their performances when operated for a long time in such a state that an excess current or overcurrent is applied thereto due to an overload or the like. Therefore, it becomes effective to use the motor under a moderate protection by monitoring the current value applied to the motor by means of a current sensor and by restraining the electric current to the motor when the state of overcurrent is detected.
As the current sensor, there have been in popular use those of the type that a current detection resistance is provided on a path for electric current to detect a voltage across the resistance by a differential amplifier circuit. The differential amplifier circuit has a characteristic that it has an offset output of a certain level even when the differential voltage input thereto is zero. The problem is that the offset output is not definite or fixed, but rather disperses among the products due to the difference in circuit constants of the products. Further, the gain of a detected current output against the level of an input differential voltage is also an important parameter influencing the measuring accuracy of the current sensor, because it disperses among the products due to the difference in circuit characteristic. In particular, where ICs for operational amplifier are incorporated into the circuit configuration, the aforementioned problem tends to occur because the dispersion in the IC characteristic is relatively large among the products.
Where the dispersion in the offset or gain of a current detection output from the current sensor gives rise to the problems as mentioned above, it has been practice to harmonize the respective characteristics of the products with one another by adding parts such as resistances, condensers and the like around the differential amplifier circuit. However, this practice is very laborious in adding the parts as well as in doing adjustment and leads a higher manufacturing cost in consequence.
Accordingly, it is a primary object of the present invention to provide an improved vehicle steering control system capable of reliably detecting the overcurrent of a drive motor by the use of a current sensor without being influenced by the dispersion in output characteristic of such current sensors as used each for the system, so that the drive motor for driving a wheel steering shaft can be protected reliably.
Briefly, according to the present invention, there is provided a vehicle steering control system of the type wherein a steering angle to be given to a wheel steering shaft is determined in dependence on a steering angle given to a handle shaft for steering operation and the driving state of a vehicle so that the wheel steering shaft is rotationally driven by a steering shaft drive motor to be given the determined steering angle. The control system comprises a current sensor including a current detection resistance provided on a motor power supply path leading from an vehicle-mounted battery to a driver for the steering shaft drive motor for detecting a current applied to the steering shaft drive motor and a differential amplifier circuit for outputting a voltage across the current detection resistance as a current detection signal. The control system further comprises offset compensation information storage means for storing offset compensation information used in offset-compensating the output of the current sensor, the offset compensation information being prepared based on a measuring value which is obtained, in advance of the actual use of the system, by measuring an offset output generated by the differential amplification circuit when the voltage across the current detection resistance is zero; and offset compensation means for compensating the output of the current sensor based on the offset compensation information during the actual use of the system.
With this construction, the offset output characteristic of the current sensor for detecting the electric current applied to the steering shaft drive motor is measured prior to the actual use or incorporation into the vehicle steering control system (i.e., prior to the shipment of the control system), and offset compensation information is prepared based on the measured values to be stored in memory means (e.g., a ROM capable of performing write-in operation at least once, such as PROM (Programmable ROM)). Then, during the actual use (i.e., after shipment), the output of the current sensor is compensated for the offset compensation information. As a result, the manufacturing cost of the control system can be reduced because it becomes unnecessary to individually adjust the circuit coefficients of differential amplifier circuits used in such systems for offset adjustment and to add parts for adjustment to each of such control systems. Further, since the offset compensation information is prepared by actually measuring the offset level for each of the control systems to be shipped, the output of the current sensor having been so compensated is highly precise. For the reasons above, the influences caused by the product-by-product dispersion in output characteristic among the current sensors can be relived effectively, and thus, it can be achieved to reliably detect the overcurrent or the like by the use of the current sensor, so that the motor for driving the wheel steering shaft can be protected reliably.
In another aspect of the present invention, there is provided a vehicle steering control system of the type wherein a steering angle to be given to a wheel steering shaft is determined in dependence on a steering angle given to a handle shaft for steering operation and the driving state of a vehicle so that the wheel steering shaft is rotationally driven by a steering shaft drive motor to be given the determined steering angle. The control system comprises a current sensor including a current detection resistance provided on a motor power supply path leading from a vehicle-mounted battery to a driver for the steering shaft drive motor for detecting a current applied to the steering shaft drive motor and a differential amplification circuit for outputting a voltage across the current detection resistance as a current detection signal. The control system further comprises current gain compensation information storage means for storing current gain compensation information which is determined using a group of the output value of the current sensor and the output value of a reference current measuring system, the both of the output values being measured in advance of the actual use of the system by applying a voltage from a predetermined measuring power supply on the current detection resistance; and current compensation means for compensating the output of the current sensor based on the current gain compensation information during the actual use of the system.
It might be the case that the differential amplifier circuit for compensating the current sensor causes the gain of a current detection output against an input differential voltage level to disperse among the products due to the dispersion in circuit characteristic of the products. To obviate this drawback, in this aspect of the present invention, prior to the actual use of the system, the measuring power supply voltage is set to a constant value, in which state the output value of the current sensor and the output value of the reference current measuring system are measured independently of each other. By this measurement, it can be grasped how much rate the output value of the current sensor has gone wrong, due to the influence of the foregoing gain dispersion, with respect to the output of the reference current measuring system which has a higher preciseness in current detection. Therefore, the current gain compensation information is prepared based on the two measured outputs. During the actual use thereafter, the output of the current sensor is compensated based on the current gain compensation information. As a result, the manufacturing cost of the control system can be reduced because it becomes unnecessary to individually adjust the circuit coefficients of differential amplifier circuits used in such systems for offset adjustment and to add parts for adjustment to each of such control systems. Further, since the offset compensation information is prepared by actually measuring the offset level for each of the control systems to be shipped, the output of the current sensor having been so compensated is highly precise. For the reasons above, the influences caused by the product-by-product dispersion in output characteristic among the current sensors can be relieved effectively, and it can therefore be achieved to reliably detect the overcurrent or the like by the use of the current sensor, so that the motor for driving the wheel steering shaft can be protected reliably.